Polypropylene copolymers and method of preparing polypropylene copolymers

ABSTRACT

Polypropylene heterophasic copolymers are produced having increased impact strength through the use of controlled rheology techniques by the addition of a peroxide at conditions which increase the deactivation or half life of the peroxide. The increased half life slows down the vis-breaking process and allows better dispersion of rubber particles within the polymer. In this way, copolymers having a high melt flow can be prepared while obtaining high impact strength and lower stiffness values, without the need for additional elastomeric modifiers.

TECHNICAL FIELD

[0001] The invention relates generally to polypropylene copolymers, andmore particularly to polypropylene heterophasic copolymers modified withperoxide to improve the impact strength of such copolymers.

BACKGROUND

[0002] Polypropylene heterophasic copolymers are typically made up ofthree components. These include a polypropylene homopolymer, a rubberyethylene propylene bipolymer, and a crystalline ethylene-rich ethylenepropylene bipolymer. The typical heterophasic morphology of thesepolymers consists of the rubbery ethylene propylene bipolymer beingdispersed as generally spherical domains within the semi-crystallinepolypropylene homopolymer matrix.

[0003] Polypropylene copolymers are used in a variety of applicationsand products. In many instances, it is particularly important for thecopolymers to have good impact strength characteristics. Polypropylenecopolymers can be modified to improve the copolymers impact strength.This can be done through the use of elastomeric modifiers or withperoxides. When using elastomeric modifiers, the elastomeric modifiersare melt blended with the polypropylene copolymer, with the increasedelastomer content typically contributing to a higher impact strength.Examples of elastomeric modifiers include ethylene propylene rubber(EPR) and ethylene propylene diene monomer (EPDM) rubber.

[0004] The use of peroxides to modify polypropylene polymers is alsoknown. WO-95/11938 discloses a process of modifying copolymers bycontacting them with a peroxide compound containing an activatedunsaturated group and an acid group in the presence of a polymerreinforcing material, or prior to the addition of a polymer reinforcingmaterial. The primary object of that invention was to modify copolymersin order to introduce an adhesion promoting functional group and toimprove their properties. The resulting modified copolymers haveimproved impact strength, flexural strength, tensile strength andelongation at break, increased melt flow index and the other propertiesequal to those of the unmodified impact copolymers.

[0005] WO-97/49759 discloses a process for enhancing the melt strengthof a propylene copolymer by the steps of mixing an initiator with thepropylene copolymer at a temperature below the decompositiontemperature; and then heating the mixture above the initiatordecomposition temperature in order to decompose the initiator before thepolymer has melted and in order to react the radicals created by thedecomposition with the polymer.

[0006] WO-96/03444 discloses a process for modifying copolymers bycontacting these with an organic cyclic ketone peroxide. Cyclic ketoneperoxides have been found particularly efficient in the modificationprocesses. They have been employed in the degradation of polyolefins,the cross-linking of polyolefins, the dynamic cross-linking of blends ofelastomers and thermoplastic polymers, the grafting of monomers ontopolymers, or the functionalization of polyolefins. The resultingmodified copolymers had a larger melt flow index, a lower weight averagemolecular weight and a narrower molecular weight distribution than thestarting copolymers, while keeping an adequate melt strength.

[0007] WO-96/20247 discloses cross-linked polymer compositions ofpropylene-ethylene copolymer and ethylene-a-olefin copolymer prepared bymelting and kneading the constituents in the presence of a radicalforming agent, a cross-linking agent and eventually a peroxideinhibitor. These compositions were characterized by a high impactstrength and a high flexural modulus.

[0008] EP-0,208,330 discloses a propylene polymer composition withincreased whitening resistance and increased impact strength, obtainedby addition of an ester, in the presence of peroxide, during extrusion.

[0009] While the aforementioned methods of modifying polymers are known,new techniques for yielding improved polypropylene heterophasiccopolymers with high flow and impact strength are needed.

SUMMARY

[0010] A method of preparing controlled rheology heterophasicpolypropylene copolymers to increase impact strength of such copolymersis provided. The method comprises introducing a heterophasicpolypropylene copolymer into an extruder along with a peroxide. Theconditions within the extruder are such that the half life of theperoxide is increased by a factor of at least 2 compared to a half lifeof the peroxide under a normal extruder condition. The peroxide isallowed to degrade the polypropylene copolymer so that the copolymerobtained has a notched Izod impact strength that is increased by atleast 50% compared to the same copolymer extruded with the peroxideunder the normal extruder condition.

[0011] In one particular embodiment, a heterophasic polypropylenecopolymer is prepared by introducing a heterophasic polypropylenecopolymer into an extruder along with a peroxide under conditionswherein the temperature within the extruder is from about 150° C. toabout 215° C., and wherein the half life of the peroxide is increased bya factor of at least 2 compared to a half life of the peroxide under anormal extruder condition in which the extruder temperature is greaterthan 215° C. The peroxide is allowed to degrade the polypropylenecopolymer so that the copolymer obtained has a notched Izod impactstrength that is increased by at least 50% compared to the samecopolymer extruded with the peroxide under the normal extrudercondition. The degraded polypropylene copolymer has a melt flow index ofgreater than about 5 g/10 min.

[0012] In other specific embodiments, the polypropylene copolymer mayhave an ethylene content of from about 5 to 20% by weight, and may havean undegraded melt flow index of from about 0.05 g/10 min to about 5g/10 min. The peroxide used may be a linear peroxide or a cyclic ketoneperoxide, and may be introduced along with the copolymer in an amount offrom about 0.005 wt % to about 0.5 wt %.

BRIEF DESCRIPTION OF THE FIGURES

[0013] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying figures, inwhich:

[0014]FIG. 1 is a photo of a microscopic sample at 4000× enlargement ofa polypropylene heterophasic copolymer extruded under normal conditionsutilizing a linear peroxide;

[0015]FIG. 2 is a photo of a microscopic sample at 4000× enlargement ofa polypropylene heterophasic copolymer extruded under normal conditionsutilizing a cyclic ketone peroxide;

[0016]FIG. 3 is a photo of a microscopic sample at 4000× enlargement ofa polypropylene heterophasic copolymer extruded under “cold” conditionsutilizing a linear peroxide, in accordance with the present invention;and

[0017]FIG. 4 is a photo of a microscopic sample at 4000× enlargement ofa polypropylene heterophasic copolymer extruded under “cold” conditionsutilizing a cyclic ketone peroxide, in accordance with the presentinvention.

DETAILED DESCRIPTION

[0018] Polypropylene copolymers modified using controlled rheologytechniques produces a higher impact polymer with lower stiffness values.The treatment of polypropylene with peroxide generally produces amodified polymer. Peroxide radicals can cause chain scission, resultingin shorter polymer chains, which results in an increase in the melt flowindex of the polymer. Such modification also causes a decrease in theflexural modulus versus non-degraded polymer of similar final melt flowindex. It has been found, however, that further improvements can beattained in impact strength in polypropylene heterophasic copolymersmodified with peroxides during the controlled rheology process byadjusting the conditions under which the controlled rheology is carriedout. Specifically, by slowing deactivation of the peroxide, impactcopolymers with higher impact strength and lower stiffness values can beattained, while achieving the desired final melt flow characteristics.

[0019] This decomposition is typically measured in terms of half-life.The half-life of peroxide is defined as the time required to decomposeone half of the peroxide molecules at a given temperature. Less reactiveperoxide will therefore have a longer half life than more reactiveperoxides.

[0020] Slowing decomposition of the peroxide during controlled rheologypolymer modification slows down the vis-breaking processes. This allowsthe polymer fluff to remain at a higher viscosity for longer periods oftime during extrusion. It is believed that by maintaining the polymerviscosity at higher levels during extrusion, the rubber phase of thepolypropylene copolymer is better dispersed, which in turn results in ahigher impact strength for the same polymer modified with peroxidehaving shorter decomposition times.

[0021] The preferred polypropylene heterophasic copolymers are thoseprepared by copolymerising propylene with ethylene in the amounts offrom about 80 to about 95% by weight of propylene and from about 5 toabout 20% by weight ethylene. Preferably, the polypropylene copolymer isthat prepared using a controlled morphology catalyst that producesethylene-propylene bipolymer domains dispersed in a semi-crystallinepolypropylene matrix. Examples of such catalysts include Ziegler-Nattaand metallocene catalysts commonly employed in the polymerization ofpropylene. The polypropylene copolymers generally consist of threecomponents. These include a propylene homopolymer, a rubberyethylene-propylene bipolymer and a crystalline ethylene-richethylene-propylene bipolymer. The amount and properties of thecomponents are controlled by the process conditions and the physicalproperties of the resulting material are correlated to the nature andamount of the three components. In the present invention, the preferredamount of ethylene is from about 5% to about 20% by weight, morepreferably from about 7 to about 15% by weight, and still morepreferably from about 11 to about 14% by weight.

[0022] The polymerization reaction may be carried out in a two-reactorconfiguration in which the catalyst and propylene are charged into afirst loop reactor equipped with a circulation pump. Preferredtemperatures within the loop reactor are from 60 to 80° C., withpressures ranging from 35 to 40 bars. The liquid propylene monomer isused as a suspension medium. Within the reactor, the propylenehomopolymer is produced on the surface of the catalyst particles. Thepropylene polymer-coated catalyst grains are then transferred to one ormore secondary gas-phase reactors with a fluidized bed. Ethylene isadded in order produce an ethylene-propylene rubber component.

[0023] The resultant polypropylene heterophasic copolymer fluff orpowder can then be processed through controlled rheology techniques withthe addition of peroxide in accordance with the invention. Additionally,non-modified or partially modified polymer pellets may also be processedin accordance with the invention to achieve similar results.

[0024] Modification or degradation of the polypropylene heterophasiccopolymer is carried out in an extruder, in which peroxide is added, inorder to increase the flow characteristics of the polymer. Additionally,other additives, such as stabilizers, antioxidants, nucleatingadditives, acid neutralizers, anti-static agents, lubricants, fillermaterials, etc., which are well known to those skilled in the art, mayalso be combined with the propylene copolymer within the extruder.

[0025] The particular choice of peroxide is not necessarily criticalprovided it achieves the necessary vis-breaking to produce the desiredfinal propylene copolymer properties. Particularly effective are thelinear organic peroxides. Preferred linear peroxides are thoserepresented by the formulae below:

R₁—O—O—R₂  (1)

R₁—O—OR₂—O—O—R₃  (2)

[0026] where R₁-R₃ are independently selected from the group consistingof hydrogen, C₁-C₂₀ alkyl, C₃-C₂₀ cycloalkyl, C₆-C₂₀ aryl, C₇-C₂₀aralykl, C₇-C₂₀ alkylaryl, which groups may include linear or branchedalkyl moieties; and each R₁-R₃ may be optionally substituted with one ormore groups selected from hydroxy, C₁-C₂₀ alkoxy, linear or branchedC₁-C₂₀ alkyl, C₆-C₂₀ aryloxy, halogen, ester carboxy, nitrile and amino.

[0027] Preferably, those linear peroxides having at least two peroxidegroups are employed, as illustrated in Equation 2 above. One such linearperoxide is 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, which has thestructural formula:

[0028] and which is commercially available as LLPEROX 101, from ATOFINAChemicals, Inc. This peroxide has a level of active oxygen on the orderof about 10% by weight. As supplied, the additive is about 92% assay forthe active component. The peroxide available as LUPEROX 101 ischaracterized by a half life (t_(1/2)) of about 233 sec. at 160° C.

[0029] Other suitable peroxides are the cyclic ketone peroxides, such asthose disclosed in WO-96/03444, which is herein incorporated byreference. The preferred cyclic ketone peroxides are those representedby the general formulae:

[0030] where R₁-R₁₀ are independently selected from the group consistingof hydrogen, C₁-C₂₀ alkyl, C₃-C₂₀ cycloalkyl, C₆-C₂₀ aryl, C₇-C₂₀aralkyl, C₇-C₂₀ alkylaryl, which groups may include linear or branchedalkyl moieties; and each of R₁-R₁₀ may be optionally substituted withone or more groups selected from hydroxy, C₁-C₂₀ alkoxy, linear orbranched C₁-C₂₀ alkyl, C₆-C₂₀ aryloxy, halogen, ester carboxy, nitrileand amino. The term “ketone peroxide,” as used herein, refers to thoseperoxides derived from either ketones or aldehydes.

[0031] The preferred cyclic ketone peroxide is that containing at leasttwo peroxide groups. A suitable cyclic ketone peroxide is3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane, which iscommercially available as TRIGONOX 301, available from Akzo NobelChemicals B.V. This latter molecule has three peroxide groups and arelatively small number of carbon atoms and thus a level of activeoxygen on the order of about 18% by weight. TRIGONOX 301 is a solutionof about 40% of 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane inISOPAR M diluent and thus has about 7.5% by weight of active oxygen. Thecyclic peroxide available as TRIGONOX 301 has a half life of about 889sec. at 160° C.

[0032] The peroxides are typically used in copolymer product in theamount of from about 0.005 wt % to about 0.5 wt %, and more typicallyfrom about 0.02 wt % to about 0.15 wt %. Depending on the particularperoxide employed, this amount may vary due to the amount of availableactive oxygen of the peroxide. The amount of peroxide used will alsovary depending upon the amount of vis-breaking desired. This is basedupon the initial melt flow of the polymer and the desired final meltflow.

[0033] As noted earlier, improvements in impact strength forpolypropylene heterophasic copolymers have been observed by slowing thedecomposition or increasing the half life of the peroxide duringdegradation. This is accomplished through a reduction in extrusiontemperatures. Alternatively, selection of a peroxide having an increasedor longer half life than would otherwise be chosen may also be employedwhere given extrusion conditions are necessary. Normal extrusiontemperatures for most controlled rheology of heterophasic copolymers areusually from about 450° F. to about 550° F., but may be hotter dependingupon the product being processed. By significantly reducing thesetemperatures, improvements in impact strength can be achieved. Asignificant increase in impact strength can be obtained, as measured bynotched Izod testing methods, when extruder temperatures are reducedsubstantially to thereby increase the half life of the peroxide.Specifically, when temperatures are reduced to increase half life timesof the peroxides by about two to about 100 times those half life timesobtained under conventional extruder temperatures to achieve the samedegree of vis-breaking, a 50% to 200%, or greater, increase in notchedIzod impact strength can be obtained for the polypropylene heterophasiccopolymers.

[0034] The polypropylene heterophasic copolymer is extruded attemperatures to maintain the material in a molten state, but which arereduced from those used in conventional controlled rheology processes.Thus, extrusion temperatures may range anywhere from the minimumtemperature to maintain the copolymer in a molten state up to about 215°C., to thereby increase the deactivation of the peroxide employed. Whensuch temperatures are employed, at least some amount of the peroxidewill usually remain unconsumed within the extruded copolymer. Typically,the temperatures will range from about 150° C. to about 215° C., withfrom about 160° C. to about 190° C. being preferred.

[0035] The melt flow index of the initial polypropylene heterophasiccopolymer fluff or powder is typically from about 0.05 g/10 min to about5 g/10 min, more typically from about 0.5 g/10 min to about 5 g/10 min,as determined by ASTM D-1238, Condition L. Unless otherwise indicated,all melt flow indices presented are measured according to ASTM D-1238,Condition L. The final extruded impact copolymer produced according tothe present invention will typically have a melt flow index of greaterthan about 5 g/10 min. The method of the invention has particularapplication in modifying or degrading those impact copolymers to a finalextruded melt flow index of from about 0.5 g/10 min to about 200 g/10min, with a range of 5 g/10 min to about 60 g/10 min being preferred.

[0036] Izod notched impact strengths in the copolymers modified andextruded according to the present invention are greater than 8 ft-lb/inas measured at room temperature.

[0037] The copolymers produced in accordance with the present inventionhave particular application to those products requiring a polymer meltindex of greater than about 10 g/10 min, high impact strength, and ahigh flexural modulus. These products are typically produced by furthermolding the melted copolymer into a desired shape. Examples of productsthat can be molded from the high impact copolymers produced inaccordance with the present invention include various storagecontainers, such as crates, yogurt and ice cream containers, storagebins, pails, medical waste containers and thin-walled packaging. Otherarticles may include suitcases, home and garden articles, automotivecomponents, batteries, and a variety of other articles.

[0038] The following examples serve to further illustrate the invention.

EXAMPLE 1

[0039] A polypropylene heterophasic copolymer fluff having a melt flowindex of 0.9 g/10 min and an ethylene content of 10.4% by weight wasused as a starting material. The copolymer was blended with additives ina high intensity mixer prior to extrusion. The additives consisted of0.10% by weight of a blend of phenolic and phosphite antioxidants; and0.04% by weight of acid neutralizer. The samples were compounded on a 1¼inch Welex extruder at 150 rpm, and through a 100 mesh screenpack. Forcomparison, both “hot” and “cold” extruder temperatures were used. Thehot extruder temperature profile was roughly 445° F. (230° C.), with thecold extruder temperature profile being roughly 365° F. (185° C.). Bothlinear and cyclic ketone peroxides were used in the form of LUPEROX 101and TRIGONOX 301, respectively. Because of differences in active oxygencontent between the different peroxides, different amounts were used toprovide similar final melt flow indices. The logarithmic degree ofvis-breaking (i.e. log(final MF)−log(initial MF)) remained generallyconstant for each of the peroxide samples. The following Table 1 setsforth the process conditions and results obtained. TABLE 1 Sample 1 2 34 Peroxide LUPER 101 TRIG 301 LUPER 101 TRIG 301 Fluff mf (actual) g/10min 0.9 0.9 0.9 0.9 Pellet mf (actual) g/10 min 18.9 16.9 18.5 17.3 Log(mf final) − log(mf initial) 1.32 1.27 1.31 1.28 Extruder used;Conditions 1 ¼″ 1 ¼″ 1 ¼″ 1 ¼″ normal condition normal condition coldcondition cold condition Melt Temp. (° F.) 446 Extruder Pressure (psi)316 316 1001 1303 Peroxide Amt. (wt %) 0.095 0.115 0.095 0.13 PolymerProperties Ethylene Content (wt. %) 10.4 10.4 10.4 10.4 Xylene Solubles(wt. %) 16.2 16.2 16.2 16.2 Notched Izod Impact (ft-lb/in) 2.2 2.0 9.89.1 Flex Mod. (Kpsi) 119 116 118 113

[0040] The improvement in rubber dispersion within the polymer matrix,which is believed to provide improved impact strength, can best be seenwith reference to FIGS. 1-4. FIGS. 1-4 correspond to Samples 1-4,respectively, from Table 1. FIGS. 1-4 show photos of the extrudedpolymer as seen by a microscope at 4000× enlargement. FIGS. 1 and 2 showthe polymer resulting from extrusion under normal or “hot” conditions,while FIGS. 3 and 4 show the polymer extruded under “cold” conditions.As can be seen, the rubber particles in FIGS. 1 and 2 are much largerand are not dispersed as well as the rubber particles of the samples ofFIGS. 3 and 4, which were extruded under the “cold” conditions, andwhich are much smaller and better dispersed.

EXAMPLE 2

[0041] The Izod notched impact strength was measured at temperatures of23, 10 and −20° C. for obtained melt indices of 12, 25 and 40, forextrusion temperatures of 160, 180 and 200° C. for the linear and cyclicketone peroxides available as LUPEROX 101 and TRIGONOX 301,respectively, using the samples from the same heterophasic impactcopolymer starting material. The controlled rheology polypropyleneheterophasic copolymer samples were prepared with an ethylene content of11.3% by weight and had a starting melt flow of approximately 2.0 g/10min. The results are presented in Table 2, with Izod values measured askJ/m². TABLE 2 Final Pellet Ext. Temp. = 200° C. Ext. Temp. = 180° C.Ext. Temp. = 160° C. Peroxide MFI Izod Values (kJ/m²) Izod Values(kJ/m²) Izod Values (kJ/m²) Impact Test Temp. (° C.) (g/10 min) 23 10−20 23 10 −20 23 10 −20 Luper. 101 12 19 na 6 45* 13 6 47* 13 6 Luper.101 25 20 na 8 32** 10 6 24** 10 6 Luper. 101 40 13 na 7 14  8 5 14  9 5Trig. 301 12 54* na 7 51* 43* 6 52* 42* 7 Trig. 301 25 51* na 7 48* 13 646* 13 6 Trig. 301 40 21 na 8 45* 11 6 45* 11 6

[0042] It can be seen that by adjusting the conditions during controlledrheology processes, where peroxide is added to degrade the polypropyleneheterophasic copolymer, so that the half life time of the peroxide isincreased, significant improvements in impact strength of the resultingpolymer can be achieved, without the need for additional elastomericmodifiers. The invention has particular application to heterophasiccopolymers having high melt flows, i.e. greater than 5 g/10 min, whichallows easier and faster processing, while still providing good impactstrength and stiffness values. High melt flow polymers with suchproperties allow shorter cycle times and reduced wall thickness ininjection molded articles made with such materials.

[0043] While the invention has been shown in only some of its forms, itshould be apparent to those skilled in the art that it is not solimited, but is susceptible to various changes and modifications withoutdeparting from the scope of the invention. Accordingly, it isappropriate that the appended claims be construed broadly and in amanner consistent with the scope of the invention.

We claim:
 1. A method of preparing controlled rheology heterophasicpolypropylene copolymers to increase impact strength of such copolymerscomprising introducing a heterophasic polypropylene copolymer into anextruder along with a peroxide under conditions wherein the half life ofthe peroxide is increased by a factor of at least 2 compared to a halflife of the peroxide under a normal extruder condition, and allowing theperoxide to degrade the polypropylene copolymer so that the copolymerobtained has a notched Izod impact strength that is increased by atleast 50% compared to the same copolymer extruded with the peroxideunder the normal extruder condition.
 2. The method of claim 1, wherein:the extruder temperature is from about 150° C. to about 215° C.
 3. Themethod of claim 1, wherein: the degraded polypropylene copolymer has amelt flow index greater than about 5 g/10 min.
 4. The method of claim 1,wherein: the undegraded polypropylene copolymer has a melt flow index offrom about 0.05 to about 5 g/10 min.
 5. The method of claim 1, wherein:the polypropylene copolymer has an ethylene content of from about 5 to20% by weight.
 6. The method of claim 1, wherein: the peroxide is2,5-dimethyl-2,5-di(t-butylperoxy)hexane.
 7. The method of claim 1,wherein: the peroxide is introduced along with the polypropylenecopolymer in an amount of from about 0.005 wt % to about 0.5 wt %. 8.The method of claim 1, further comprising: molding the extrudedpolypropylene copolymer into a desired article.
 9. The polypropylenecopolymer prepared by the method of claim
 1. 10. The polypropylenecopolymer of claim 1, wherein: the peroxide is a cyclic ketone peroxide.11. The polypropylene copolymer of claim 1, wherein: the peroxide is alinear peroxide.
 12. The polypropylene copolymer of claim 1, wherein:the half life of the peroxide is increased by a factor ranging from 2 to100.
 13. An article prepared by the method of claim
 8. 14. Aheterophasic polypropylene copolymer prepared by introducing aheterophasic polypropylene copolymer into an extruder along with aperoxide under conditions wherein the temperature within the extruder isfrom about 150° C. to about 215° C. and wherein the half life of theperoxide is increased by a factor of at least 2 compared to a half lifeof the peroxide under a normal extruder condition in which the extrudertemperature is greater than 215° C., and allowing the peroxide todegrade the polypropylene copolymer so that the copolymer obtained has anotched Izod impact strength that is increased by at least 50% comparedto the same copolymer extruded with the peroxide under the normalextruder condition, the degraded polypropylene copolymer having a meltflow index of greater than about 5 g/110 min.
 15. The polypropylenecopolymer of claim 14, wherein: the peroxide is a cyclic ketoneperoxide.
 16. The polypropylene copolymer of claim 14, wherein: theperoxide is a linear peroxide.
 17. The polypropylene copolymer of claim14, wherein: the undegraded polypropylene copolymer has a melt flowindex of from about 0.05 to about 5 g/10 min.
 18. The polypropylenecopolymer of claim 14, wherein: the polypropylene copolymer has anethylene content of from about 5 to 20% by weight.
 19. The polypropylenecopolymer of claim 14, wherein: the peroxide is3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane.
 20. Thepolypropylene copolymer of claim 14, wherein: the peroxide is2,5-dimethyl-2,5-di(t-butylperoxy)hexane.
 21. The polypropylenecopolymer of claim 14, wherein: the peroxide is introduced along withthe polypropylene copolymer in an amount of from about 0.005 wt % toabout 0.5 wt %.
 22. An article prepared with the degraded polypropylenecopolymer of claim
 14. 23. A method of preparing controlled rheologyheterophasic polypropylene copolymers to increase impact strength ofsuch copolymers comprising introducing a heterophasic polypropylenecopolymer into an extruder along with a peroxide under conditions wherethe temperature within the extruder is from about 150° C. to about 215°to obtain a copolymer having an increase of from 50% or greater innotched Izod impact strength compared to the copolymer degraded with theperoxide at temperatures higher than 215° C.